As easy-to-access and easy-to-produce hydrocarbon resources are depleted, there is an increased demand for more advanced recovery procedures. One such procedure is steam assisted gravity drainage (SAGD), a procedure that utilizes steam in conjunction with two spaced apart wellbores. Specifically, SAGD addresses the mobility problem of heavy oil in a formation through the injection of high pressure, high temperature steam into the formation. This high pressure, high temperature steam reduces the viscosity of the heavy oil in order to enhance extraction. The injection of steam into the formation occurs from a first wellbore (injector) that is drilled above and parallel to a second wellbore (producer). As the viscosity of the heavy oil in the formation around the first wellbore is reduced, the heavy oil drains into the lower second wellbore, from which the oil is extracted. Preferably, the two wellbores are drilled at a distance of only a few meters from one other. The placement of the injector wellbore needs to be achieved with very small margin in distance. If the injector wellbore is positioned too close to the producer wellbore, the producing wellbore would be exposed to very high pressure and temperature. If the injector wellbore is positioned too far from the producer wellbore, the efficiency of the SAGD process is reduced. In order to assist in ensuring that the second wellbore is drilled and positioned as desired relative to the first wellbore, a survey of the two wellbores in the formation is often conducted. These surveying techniques are traditionally referred to as ranging.
One solution that has been employed in ranging is to deploy an electromagnetic field gradiometer tool in the wellbore being drilled and inject an alternating current in the casing of the target wellbore to produce a time-varying magnetic field about the casing. The gradiometer tool includes coils or other sensors that generate electromotive force via inductance in the presence of the time-varying magnetic field, which may be processed for ranging. However, imperfections in gradiometer tool manufacture can affect response voltages of its sensors to the magnetic field gradient. Because precise response voltages are required for the effective ranging analysis, such tool imperfections must be compensated. Compensation may be effected by mathematical characterization of the overall tool response to a current loop of given parameters in a laboratory environment prior to use in the field.